The present invention relates to lasers, and more particularly to a gas discharge laser that utilizes a new type of discharge that offers an improved efficiency over that which has heretofore been achieved.
A laser is a device that generates coherent, nearly single-wavelength (and frequency), highly directional electromagnetic radiation emitted somewhere in the range from the submillimeter through ultraviolet and x-ray wavelengths. The word laser is an acronym for "Light Amplification by Stimulated Emission of Radiation."
Quantum theory shows that matter can exist in only certain allowed energy levels or states. In thermal equilibrium, lower energy states of matter are preferentially populated, since occupation probability is proportional to e.sup.E/kT, where E is the state energy, T is the temperature, and k is the Boltzmann constant. An excited state will normally decay spontaneously to a lower energy state, emitting a quantum or wave packet of electromagnetic radiation (photon) with transition frequency .upsilon.=.DELTA.E/h, where .DELTA.E is the energy difference between the two states and h is Planck's constant. The presence of radiation at frequency .upsilon. can cause a transition from the lower energy state to the upper energy state with the absorption of a photon and a corresponding decrease in the electromagnetic field energy. A transition from the upper state to the lower state can also be induced by the radiation at the same time a photon wave packet is emitted coherently with (i.e., having the same frequency as) the stimulating radiation wave. Such stimulated emission process is the reverse of the absorption process. If the matter can be forced out of thermal equilibrium to a sufficient degree, so that the upper state has a higher population than the lower state (a condition known as "population inversion"), then more stimulated emission than absorption occurs, leading to the coherent growth (amplification or gain) of the electromagnetic wave at the transition frequency.
A laser generally requires three components for its operation: (1) an active medium with energy levels that can be selectively populated; (2) a pumping process to produce population inversion between some of these energy levels; and (3) a resonant electromagnetic cavity structure containing the active medium, which serves to store the emitted radiation and provide feedback to maintain the coherence of the electromagnetic field. In a continuously operating laser, coherent radiation will build up in the cavity to a level required to balance the stimulated emission process with the cavity and medium losses. The system is then said to be "lasing", and radiation is emitted in a direction defined by the cavity.
One type or class of lasers uses a gas as the active or lasing medium. External energy is applied to the gas in order to initiate a discharge that causes inversion to occur between some of the constituents of the gas. Gas lasers advantageously provide high output power, needed for many laser applications. Disadvantageously, gas lasers, and in particular ion lasers, are notoriously inefficient. For example, in order to produce an output power of 10-30 W using an ion laser, such as an Argon ion laser, an input power of 20-60 KW is required. Thus, while a desired high power laser output can be achieved using a gas laser, such high power output comes at great expense (very high input power). Thus, what is needed is a gas laser that provides high output power at increased efficiency.
For example, in an Ar.sup.+ laser, the population inversion occurs, as is the case for most ion lasers, through excitation of Ar ions from a ground state (nonexcited state) to a first excited state (or upper level). From the upper level there is a slow radiative decay to the ground state and to a second excited state (lower level). From the lower level there is a very rapid radiative decay back to the ground state. Excitation to the upper level from the ground state occurs through electron impact. There are no significant collisional effects on the decay rates. Rather, all decay is purely radiative. Hence, the pressure of the neutral gas in an ion laser may be much lower than is the case with non-ion gas lasers, where a higher pressure is needed to maintain a needed collisional rate. The population inversion in the argon ion laser occurs due to the slow decay of the upper lasing level, and the rapid decay from the lower level to the ground level.
As indicated, excitation to the upper level occurs due to electron impact on the argon ion. Hence, the excitation rate and therefore the spontaneous emission of the visible output depend primarily on the electron density, which equals the ion density. For typical electron temperatures, the density at which the maximum lasing action occurs is between 10.sup.14 cm.sup.-3 and 10.sup.15 cm.sup.-3. Once this density is achieved, the gain for the visible output is saturated, and the visible output power density cannot be increased. Hence, in an argon ion laser, or other ion laser where a similar inversion mechanism is employed, the key problem addressed is how to maintain an ion density of between 10.sup.14 cm.sup.-3 and 10.sup.15 cm.sup.-3 using as little input power as possible. Disadvantageously, prior art argon ion lasers have required a great deal of input power to maintain such ion density, and hence such lasers are very inefficient.
Thus, where the lasing species comprises a fully or partially ionized gas, i.e., a plasma, the available output power is highly dependent upon the plasma density. The plasma is confined within a narrow discharge tube, with the plasma being confined near the center of the discharge tube away from the walls of the discharge tube. The ions in the plasma are quickly spent (recombined to form neutral particles, or otherwise changed to a state not useful as a lasing species) whenever they come in contact with the walls of the discharge tube. Such loss of the ions or plasma into the walls of the discharge tube is referred to as "radial plasma loss". The input power to the laser depends on the radial plasma loss rate. That is, the faster the plasma is lost, the more input power is required to maintain a given density. Thus, for more efficient operation, where efficiency is defined as the output power divided by the input power, it is essential that the radial plasma loss be kept small.
Where a plasma is used as the lasing species, a certain amount of energy must be expended in order to create, and then maintain, the plasma. This is commonly done by applying a large dc voltage across spaced-apart electrodes placed within the discharge tube, which high voltage causes the gas to break down so as to create the desired ion constituents, thereby forming the plasma. Once the plasma is formed, a large dc current is allowed to flow through the plasma, which dc current sustains the lasing action (i.e., provides the energy to maintain the plasma and the needed population inversion, which inversion occurs when the density is high enough, as explained above). Disadvantageously, the large dc current also creates hydrodynamic instabilities that cause the bulk,of the plasma to blow apart and be spent in the walls of the discharge tube. As a result, the walls heat up, and must be cooled, e.g., using an externally applied coolant, to avoid thermal meltdown. All this action necessitates that additional power be put into the laser in order to create new plasma (to replace that which has been blown apart) and in order to cool the walls of the discharge tube. Such increased input power further decreases the efficiency of the laser. Hence, what is needed is an ion or plasma laser wherein the need for such additional input energy is minimized or eliminated, thereby improving the overall laser efficiency.
Additionally, it is noted that where the discharge tube is cooled, the material from which the discharge tube is made (typically quartz) experiences severe thermal cycling, thereby shortening its life, and hence shortening the life of the laser. It would be desirable if the severe thermal cycling to which the discharge tube is subjected could be reduced, thereby lowering the cooling requirements of the discharge tube, and increasing its operating life.
Further, the use of electrodes within the discharge tube of a gas laser limits the types of gases that may be placed therewithin. That is, some gases may be highly reactive, and quickly react with any conductive electrodes that may be within the discharge tube area, thereby decreasing the operating life of the laser. Hence, an electrodeless laser would be desirable, thereby effectively increasing the life of the laser and permitting more reactive gases to be used therewithin.
Another element that contributes to the inefficiencies of a gas or ion laser is the presence of neutral particles that tend to drift (be carried) with the dc current towards one end of the plasma column within the discharge tube, thereby creating a pressure differential between the ends of the plasma column which interferes with the lasing operation. Elaborate "gas return systems" are thus used in the art that remove the neutral particles from one end of the plasma column and reintroduce them at the other. Their presence further adds to the expense and complexity of the laser. What is thus needed is a gas laser wherein such gas return systems are not needed.